Laser interferometer

ABSTRACT

A laser interferometer includes a light source that emits first laser light, an optical modulator that includes a vibrator and modulates the first laser light by using the vibrator to generate second laser light including a modulated signal, a photodetector that receives interference light between third laser light including a sample signal generated by reflecting the first laser light on an object and the second laser light to output a light reception signal, and an optical path length variable section that changes an optical path length of an optical path through which the third laser light propagates.

The present application is based on, and claims priority from JPApplication Serial Number 2020-143297, filed Aug. 27, 2020, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a laser interferometer.

2. Related Art

JP-A-2007-285898 discloses a laser vibrometer, as a device for measuringthe vibration velocity of an object, that irradiates the object withlaser light and measures the vibration velocity based on the scatteredlaser light subjected to Doppler shift. In the laser vibrometer, theDoppler signal contained in the scattered laser light is extracted byusing the optical heterodyne interferometry.

Further, in the laser vibrometer disclosed in JP-A-2007-285898, a piezoelement or a quartz crystal resonator of which the vibration frequencyis variable by changing the voltage is used, in which frequencies areshifted by irradiating the vibrators with laser light. By using thelaser light including the modulated signal of which the frequency isshifted in this way as reference light, the Doppler signal isdemodulated from the scattered laser light. By using the Doppler signalobtained in this way, the vibration velocity of the object can bemeasured.

Meanwhile, JP-A-2-38889 discloses that a sine wave signal is applied toan optical modulator, a reference light beam of which the frequency isshifted in a sine wave shape with respect to a light beam and areflected light beam obtained by irradiating an object with the lightbeam are received by a photodetector, and arithmetic processing on alight reception signal is performed. Further, JP-A-2-38889 disclosesthat, even when the frequency of the reference light beam is shiftedover time, a signal derived from the object (beat signal) can beobtained, and a displacement and vibration velocity of the object can beobtained by allowing the beat signal to pass through an FM demodulationcircuit.

In the method disclosed in JP-A-2-38889, a demodulation process isperformed that splits the light reception signal into two, appliesseparate arithmetic processing, and then adds them to erase anunnecessary term of the signal and finally extract the beat signal.However, when the vibration state of the object does not satisfy apredetermined condition, there is a problem that the beat signal cannotbe demodulated with high accuracy and the displacement and vibrationvelocity of the object cannot be obtained with high accuracy.

SUMMARY

A laser interferometer according to an application example of thepresent disclosure includes: a light source that emits first laserlight; an optical modulator that includes a vibrator and modulates thefirst laser light by using the vibrator to generate second laser lightincluding a modulated signal; a photodetector that receives interferencelight between third laser light including a sample signal generated byreflecting the first laser light on an object and the second laser lightto output a light reception signal; and an optical path length variablesection that changes an optical path length of an optical path throughwhich the third laser light propagates.

A laser interferometer according to another application example of thepresent disclosure includes: a light source that emits first laserlight; an optical modulator that includes a vibrator and modulates thefirst laser light by using the vibrator to generate second laser lightincluding a modulated signal; a photodetector that receives interferencelight between third laser light including a sample signal generated byreflecting the first laser light on an object and the second laser lightto output a light reception signal; and an optical path length variablesection that changes an optical path length of an optical path throughwhich the second laser light propagates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram showing a laser interferometeraccording to an embodiment.

FIG. 2 is a schematic configuration diagram showing a sensor headsection and an optical system shown in FIG. 1.

FIG. 3 is a perspective view showing a first configuration example of anoptical modulator shown in FIG. 2.

FIG. 4 is a plan view showing a part of a second configuration exampleof the optical modulator.

FIG. 5 is a plan view showing a third configuration example of theoptical modulator.

FIG. 6 is a conceptual diagram illustrating that a plurality ofdiffracted light beams are generated when incident light is incident ina direction perpendicular to a surface of a vibrator shown in FIG. 3.

FIG. 7 is a conceptual diagram illustrating an optical modulatorconfigured such that the angle formed by the traveling direction of theincident light and the traveling direction of the reference light is180°.

FIG. 8 is a conceptual diagram illustrating an optical modulatorconfigured such that the angle formed by the traveling direction of theincident light and the traveling direction of the reference light is180°.

FIG. 9 is a conceptual diagram illustrating an optical modulatorconfigured such that the angle formed by the traveling direction of theincident light and the traveling direction of the reference light is180°.

FIG. 10 is a cross-sectional view showing an optical modulator having apackage structure.

FIG. 11 is a circuit diagram showing a configuration of a one-stageinverter oscillation circuit.

FIG. 12 is an example of an LCR equivalent circuit of a vibrator.

FIG. 13 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a first modification example.

FIG. 14 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a second modification example.

FIG. 15 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a third modification example.

FIG. 16 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a fourth modification example.

FIG. 17 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a fifth modification example.

FIG. 18 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a sixth modification example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, the laser interferometer according to the presentdisclosure will be described in detail based on the embodiments shown inthe accompanying drawings.

FIG. 1 is a functional block diagram showing a laser interferometeraccording to an embodiment.

A laser interferometer 1 shown in FIG. 1 has a sensor head section 51provided with an optical system 50, a demodulation circuit 52 to which alight reception signal from the optical system 50 is input, anoscillation circuit 54 that outputs a reference signal to thedemodulation circuit 52, and a control section 57.

1. Sensor Head Section

FIG. 2 is a schematic configuration diagram showing the sensor headsection 51 and the optical system 50 shown in FIG. 1.

The optical system 50 includes a light source 2, a polarization beamsplitter 4, a quarter wave plate 6, a quarter wave plate 8, an opticalanalyzer 9, a photodetector 10, a frequency shifter type opticalmodulator 12, an optical path length variable section 13, and a setsection 16 in which an object 14 to be measured is arranged.

The light source 2 emits emission light L1 (first laser light) having apredetermined wavelength. The photodetector 10 converts the receivedlight into an electric signal. The optical modulator 12 includes avibrator 30 and modulates the emission light L1 to generate referencelight L2 (second laser light) including a modulated signal. The setsection 16 may be provided as needed, and the object 14 to be measuredcan be arranged therein. The emission light L1 incident on the object 14to be measured is reflected as object light L3 (third laser light)including a sample signal derived from the object 14 to be measured.

The optical path of the emission light L1 emitted from the light source2 is referred to as an optical path 18. The optical path 18 is coupledto the optical path 20 by the reflection of the polarization beamsplitter 4. On the optical path 20, the quarter wave plate 8 and theoptical modulator 12 are arranged in this order from the polarizationbeam splitter 4 side. Further, the optical path 18 is coupled to anoptical path 22 by the transmission of the polarization beam splitter 4.On the optical path 22, the quarter wave plate 6, the optical pathlength variable section 13, and the set section 16 are arranged in thisorder from the polarization beam splitter 4 side.

The optical path 20 is coupled to an optical path 24 by the transmissionof the polarization beam splitter 4. On the optical path 24, the opticalanalyzer 9 and the photodetector 10 are arranged in this order from thepolarization beam splitter 4 side.

The emission light L1 emitted from the light source 2 enters the opticalmodulator 12 via the optical path 18 and the optical path 20. Further,the emission light L1 enters the object 14 to be measured via theoptical path 18 and the optical path 22. The reference light L2generated by the optical modulator 12 enters the photodetector 10 viathe optical path 20 and the optical path 24. The object light L3generated by the reflection from the object 14 to be measured enters thephotodetector 10 via the optical path 22 and the optical path 24.

Hereinafter, each section of the laser interferometer 1 will bedescribed in sequence.

1.1. Light Source

The light source 2 is a laser light source that emits an emission lightL1 with a narrow line width having coherence. When the line width isdisplayed by frequency difference, a laser light source having a linewidth of MHz band or less is preferably used. Specifically, gas laserssuch as HeNe lasers, and semiconductor laser elements such as adistributed feedback-laser diode (DFB-LD), a laser diode with fiberbragg grating (FBG-LD), or a vertical cavity surface emitting laser(VCSEL) can be exemplified.

The light source 2 preferably includes a semiconductor laser element.This makes it possible to make the light source 2 particularlyminiaturized. Therefore, the laser interferometer 1 can be made smaller.In particular, the sensor head section 51 in the laser interferometer 1,in which the optical system 50 is accommodated, can be made smaller andlighter, and thus it is also useful in that the operability of the laserinterferometer 1 can be improved.

1.2. Polarization Beam Splitter

The polarization beam splitter 4 is an optical element that splits theincident light into transmitted light and reflected light. Further, thepolarization beam splitter 4 has a function of transmitting P-polarizedlight and reflecting S-polarized light, and can divide the polarizationstate of the incident light into orthogonal components. Hereinafter, acase where the emission light L1 which is linearly polarized light andhas a ratio between the P-polarized light and the S-polarized light of,for example, 50:50 is incident on the polarization beam splitter 4 willbe considered.

As described above, the polarization beam splitter 4 reflects theS-polarized light of the emission light L1 and transmits the P-polarizedlight.

The S-polarized light of the emission light L1 reflected by thepolarization beam splitter 4 is converted into circularly polarizedlight at the quarter wave plate 8 and incident on the optical modulator12. The circularly polarized light of the emission light L1 incident onthe optical modulator 12 undergoes a frequency shift of f_(m) [Hz] andis reflected as the reference light L2. Therefore, the reference lightL2 includes a modulated signal having a modulation frequency of f_(m)[Hz]. The reference light L2 is converted into the P-polarized lightwhen the reference light L2 passes through the quarter wave plate 8again. The P-polarized light of the reference light L2 passes throughthe polarization beam splitter 4 and the optical analyzer 9 and isincident on the photodetector 10.

The P-polarized light of the emission light L1 passing through thepolarization beam splitter 4 is converted into circularly polarizedlight at the quarter wave plate 6 and incident on the object 14 to bemeasured in a moving state. The circularly polarized light of theemission light L1 incident on the object 14 to be measured undergoes aDoppler shift of f_(d) [Hz] and is reflected as the object light L3.Therefore, the object light L3 includes a frequency signal having afrequency of f_(d) [Hz]. The object light L3 is converted into theS-polarized light when the object light L3 passes through the quarterwave plate 6 again via the optical path length variable section 13. TheS-polarized light of the object light L3 is reflected by thepolarization beam splitter 4, passes through the optical analyzer 9, andis incident on the photodetector 10.

As described above, since the emission light L1 has coherence, thereference light L2 and the object light L3 are incident on thephotodetector 10 as interference light.

A non-polarization beam splitter may be used instead of the polarizationbeam splitter. In this case, since the quarter wave plate 6 and thequarter wave plate 8 are not required, the laser interferometer 1 can bemade smaller by reducing the number of parts.

1.3. Optical Analyzer

Since the S-polarized light and the P-polarized light that areorthogonal to each other are independent of each other, interferencedoes not appear by simply superimposing them. Therefore, a light waveobtained by superimposing the S-polarized light onto the P-polarizedlight is passed through the optical analyzer 9 tilted by 45° withrespect to both S-polarized light and P-polarized light. By using theoptical analyzer 9, light of the components common to each other can betransmitted to cause interference. As a result, in the optical analyzer9, the modulated signal and the sample signal interfere with each other,and interference light having a frequency of f_(m)−f_(d) [Hz] isgenerated.

1.4. Photodetector

The reference light L2 and the object light L3 are incident on thephotodetector 10 via the polarization beam splitter 4 and the opticalanalyzer 9. Thereby, the reference light L2 and the object light L3interfere with each other by optical heterodyne, and the interferencelight having a frequency of f_(m)−f_(d) [Hz] is incident on thephotodetector 10. By demodulating the sample signal from theinterference light by the method to be described later, the movement ofthe object 14 to be measured, that is, the velocity and the vibrationcan be finally obtained. Examples of the photodetector 10 include aphotodiode and the like.

1.5. Optical Modulator

FIG. 3 is a perspective view showing a first configuration example ofthe optical modulator 12 shown in FIG. 2.

1.5.1. Outline of Optical Modulator According to First ConfigurationExample

The frequency shifter type optical modulator 12 has an opticalmodulation resonator 120. The optical modulation resonator 120 shown inFIG. 3 includes a plate-shaped vibrator 30 and a substrate 31 thatsupports the vibrator 30.

The vibrator 30 is made of a material that repeats a mode of vibratingso as to be distorted in a direction along a surface by applying anelectric potential. In the present embodiment, the vibrator 30 is aquartz crystal AT resonator that vibrates by thickness sliding along avibration direction 36 in the high frequency region of the MHz band. Adiffraction grating 34 is formed on the surface of the vibrator 30. Thediffraction grating 34 has a structure in which a plurality of lineargrooves 32 are arranged at regular intervals.

The substrate 31 has a front surface 311 and a back surface 312 having afront and back relationship with each other. The vibrator 30 is arrangedon the front surface 311. Further, the front surface 311 is providedwith a pad 33 for applying an electric potential to the vibrator 30. Onthe other hand, the back surface 312 is also provided with a pad 35 forapplying an electric potential to the vibrator 30.

The size of the substrate 31 is, for example, about 0.5 mm or more and10.0 mm or less on the long side. The thickness of the substrate 31 is,for example, about 0.10 mm or more and 2.0 mm or less. As an example,the substrate 31 has a shape of a square with a side of 1.6 mm and has athickness of 0.35 mm.

The size of the vibrator 30 is, for example, about 0.2 mm or more and3.0 mm or less on the long side. The thickness of the vibrator 30 is,for example, about 0.003 mm or more and 0.5 mm or less.

As an example, the vibrator 30 has a shape of a square with a side of1.0 mm and has a thickness of 0.07 mm. In this case, the vibrator 30oscillates at a basic oscillation frequency of 24 MHz. The oscillationfrequency can be adjusted in the range of 1 MHz to 1 GHz by changing thethickness of the vibrator 30 and even considering overtones.

In FIG. 3, the diffraction grating 34 is formed on the entire surface ofthe vibrator 30, but it may be formed only on a part of the surface.

The intensity of the optical modulation by the optical modulator 12 isgiven by a dot product of a difference wave vector between a wave vectorof the emission light L1 incident on the optical modulator 12 and a wavevector of the emission light L2 emitted from the optical modulator 12and a vector of the vibrator 30 in the vibration direction 36. In thepresent embodiment, since the vibrator 30 vibrates by thickness slidingand this vibration is in-plane vibration, optical modulation cannot beperformed even if light is incident perpendicularly to the surface ofthe vibrator 30 alone. Therefore, in the present embodiment, byproviding the diffraction grating 34 in the vibrator 30, opticalmodulation can be performed by a principle to be described later.

The diffraction grating 34 shown in FIG. 3 is a blazed diffractiongrating. The blazed diffraction grating is one in which thecross-sectional shape of the diffraction grating is stepped. The lineargroove 32 of the diffraction grating 34 is provided such that itsextending direction is orthogonal to the vibration direction 36.

When a drive signal S1 is supplied from the oscillation circuit 54 shownin FIG. 1 to the vibrator 30 shown in FIG. 3 (an AC voltage is applied),the vibrator 30 oscillates. The electric power (driving power) requiredfor the oscillation of the vibrator 30 is not particularly limited, butis as small as about 0.1 μW to 100 mW. Therefore, the drive signal S1output from the oscillation circuit 54 can be used to oscillate thevibrator 30 without amplification.

Further, since the optical modulator in the related art needs astructure for maintaining the temperature of the optical modulator, itis difficult to reduce the volume. Further, since the power consumptionof the optical modulator is large, there is a problem that it isdifficult to reduce the size and power consumption of the laserinterferometer. On the other hand, in the present embodiment, since thevolume of the vibrator 30 is very small and the electric power requiredfor oscillation is also small, it is easy to reduce the size and powerconsumption of the laser interferometer 1.

1.5.2. Method of Forming Diffraction Grating

The method of forming the diffraction grating 34 is not particularlylimited, but as an example, a method may be provided in which the moldis formed by a method using a mechanical engraving type (ruling engine)and the groove 32 is formed on an electrode formed on the surface of thevibrator 30 of the quartz crystal AT resonator by the nanoimprintmethod. Here, the reason why it is formed on the electrode is that inthe case of the quartz crystal AT resonator, in principle, high-qualitythickness sliding vibration can be generated on the electrode. Thegroove 32 is not limited to being formed on the electrode, but may beformed on the surface of the material of the non-electrode portion.Further, instead of the nanoimprint method, a processing method byexposure and etching, an electron beam drawing lithography method, afocused ion beam processing method (FIB), or the like may be used.

Further, a diffraction grating of a resist material may be formed on thechip of the quartz crystal AT resonator, and a metal film or a mirrorfilm made of a dielectric multilayer film may be provided therein. Byproviding the metal film or the mirror film, the reflectance of thediffraction grating 34 can be increased.

Further, the metal film or the mirror film may be formed on a processedsurface after forming the resist film is on the chip or the wafer of thequartz crystal AT resonator, performing etching processing, and thenremoving the resist film. In this case, since the resist material isremoved, the influence of moisture absorption of the resist material orthe like is eliminated, and the chemical stability of the diffractiongrating 34 can be improved. Further, by providing a highly conductivemetal film such as Au or Al, it can also be used as an electrode fordriving the vibrator 30.

The diffraction grating 34 may be formed by using a technique such asanodized alumina (porous alumina).

1.5.3. Optical Modulator According to Another Configuration Example

The vibrator 30 is not limited to the quartz crystal resonator, and maybe, for example, a Si resonator, a surface acoustic wave (SAW) device,or the like.

FIG. 4 is a plan view showing a part of a second configuration exampleof the optical modulator 12. FIG. 5 is a plan view showing a thirdconfiguration example of the optical modulator 12.

A vibrator 30A shown in FIG. 4 is a Si resonator manufactured by usingthe MEMS technique. MEMS stands for Micro Electro Mechanical Systems.

The vibrator 30A includes a first electrode 301 and a second electrode302 adjacent to each other on the same plane with a gap, a diffractiongrating mounting portion 303 provided on the first electrode 301, andthe diffraction grating 34 provided on the diffraction grating mountingportion 303. For example, the first electrode 301 and the secondelectrode 302 vibrate in the left-right direction of FIG. 4 so as torepeatedly approach and separate from each other by using anelectrostatic attraction force as a driving force. Thereby, in-planevibration can be applied to the diffraction grating 34. The oscillationfrequency of the Si resonator is, for example, about 1 kHz to severalhundred MHz.

A vibrator 30B shown in FIG. 5 is a SAW device that utilizes surfacewaves. SAW stands for a Surface Acoustic Wave.

The vibrator 30B includes a piezoelectric substrate 305, inter digitaltransducers 306 provided on the piezoelectric substrate 305, a groundelectrode 307, the diffraction grating mounting portion 303, and thediffraction grating 34. When an AC voltage is applied to the interdigital transducer 306, the surface wave is excited by the inversepiezoelectric effect. Thereby, in-plane vibration can be applied to thediffraction grating 34. The oscillation frequency of the SAW device is,for example, several hundred MHz to several GHz.

By providing the diffraction grating 34 in the above-mentioned device aswell, optical modulation can be performed by the principle to bedescribed later, as in the case of the quartz crystal AT resonator.

On the other hand, when the vibrator 30 has the quartz crystalresonator, a highly accurate modulated signal can be generated byutilizing an extremely high Q value of the quartz crystal. The Q valueis an index showing the sharpness of the peak of resonance. Further, asa feature of the quartz crystal resonator, it is not easily affected bydisturbance. Therefore, by using the modulated signal modulated by theoptical modulator 12 including the quartz crystal resonator, a samplesignal derived from the object 14 to be measured can be acquired withhigh accuracy.

1.5.4. Optical Modulation by Vibrator

Next, the principle of modulating light by using the optical modulator12 shown in FIG. 3 will be described.

FIG. 6 is a conceptual diagram illustrating that a plurality ofdiffracted light beams are generated when incident light K_(i) isincident in a direction perpendicular to the surface of the vibrator 30shown in FIG. 3.

When the incident light K_(i) is incident on the diffraction grating 34that is vibrating by thickness sliding along the vibration direction 36,a plurality of diffracted light beams K_(ns) are generated as shown inFIG. 6 due to the diffraction phenomenon. n is the order of thediffracted light K_(ns), and n=0, ±1, ±2, . . . . In FIG. 6, thediffraction grating 34 is not the blazed diffraction grating shown inFIG. 3, but a diffraction grating made by repeated irregularities as anexample of another diffraction grating.

In FIG. 6, the incident light K_(i) is incident in a directionperpendicular to the surface of the vibrator 30; however, the incidentangle is not particularly limited and may be set so that the incidentlight is obliquely incident on the surface of the vibrator 30. When thelight is incident at an angle, the traveling direction of the diffractedlight K_(ns) also changes accordingly.

Depending on the design of the diffraction grating 34, higher-orderlight of |n|≥2 may not appear. Therefore, in order to obtain a stablemodulated signal, it is desirable to set |n|=1. That is, in the laserinterferometer 1 of FIG. 2, the frequency shifter type optical modulator12 is preferably arranged so that the ±1st-order diffracted light isused as the reference light L2. With this arrangement, it is possible toachieve stabilization of measurement by the laser interferometer 1.

On the other hand, when higher-order light of |n|≥2 appears from thediffraction grating 34, the optical modulator 12 may be arranged so thatany diffracted light of ±2nd order or higher is used as the referencelight L2 instead of the ±1st-order diffracted light. Thereby,higher-order diffracted light can be used, and thus the laserinterferometer 1 can be made higher in frequency and smaller in size.

In the present embodiment, as an example, the optical modulator 12 isconfigured such that the angle formed by an entering direction of theincident light K_(i) incident on the optical modulator 12 and thetraveling direction of the reference light L2 emitted from the opticalmodulator 12 is 180°. Hereinafter, three configuration examples will bedescribed.

FIGS. 7 to 9 are conceptual diagrams illustrating the optical modulator12 configured such that an angle formed by the traveling direction ofthe incident light K_(i) and the traveling direction of the referencelight L2 is 180°.

In FIG. 7, the optical modulator 12 includes a mirror 37 in addition tothe vibrator 30. The mirror 37 is arranged so as to reflect diffractedlight K_(1s) and return it to the diffraction grating 34. At this time,the angle formed by the incident angle of the diffracted light K_(1s)with respect to the mirror 37 and the reflection angle of the mirror 37is 180°. As a result, the diffracted light K_(1s) emitted from themirror 37 and returned to the diffraction grating 34 is diffracted againby the diffraction grating 34, and travels in the direction opposite tothe traveling direction of the incident light K_(i) incident on theoptical modulator 12. Therefore, by adding the mirror 37, it is possibleto satisfy the condition that the angle formed by the entering directionof the incident light K_(i) and the traveling direction of the referencelight L2 is 180°, as described above.

By passing through the mirror 37 in this way, the reference light L2generated by the optical modulator 12 is subjected to frequencymodulation twice. Therefore, by using the mirror 37 together, thefrequency modulation of a high frequency becomes possible as comparedwith the case where the vibrator 30 alone is used.

In FIG. 8, the vibrator 30 is tilted with respect to the arrangementshown in FIG. 6. The inclination angle θ at this time is set so as tosatisfy the condition that the angle formed by the entering direction ofthe incident light K_(i) and the traveling direction of the referencelight L2 is 180°.

The diffraction grating 34 shown in FIG. 9 is a blazed diffractiongrating having a blaze angle θ_(B). Then, when the incident light K_(i)traveling at an incident angle β is incident on the diffraction grating34 with respect to a normal line N on the surface of the vibrator 30,the reference light L2 returns at the same angle as the blaze angleθ_(B) with respect to the normal line N. Therefore, by making theincident angle β equal to the blaze angle θ_(B), it is possible tosatisfy the condition that the angle formed by the entering direction ofthe incident light K_(i) and the traveling direction of the referencelight L2 is 180°, as described above. In this case, since the aboveconditions can be satisfied without using the mirror 37 shown in FIG. 7and without tilting the vibrator 30 itself as shown in FIG. 8, the laserinterferometer 1 can be further miniaturized and have a higherfrequency. In particular, in the case of a blazed diffraction grating,an arrangement satisfying the above conditions is called a “Littrowarrangement”, and there is an advantage that the diffraction efficiencyof the diffracted light can be particularly improved.

A pitch P in FIG. 9 represents the pitch of the blazed diffractiongrating, and as an example, the pitch P is 1 μm. The blaze angle θ_(B)is 25°. In this case, in order to satisfy the above conditions, theincident angle β with respect to the normal line N of the incident lightK_(i) may be set to 25°.

1.5.5. Package Structure

FIG. 10 is a cross-sectional view showing the optical modulator 12having a package structure.

The optical modulator 12 shown in FIG. 10 includes a container 70 whichis a housing, the optical modulation resonator 120 housed in thecontainer 70, and a circuit element 45 constituting the oscillationcircuit 54. The container 70 is hermetically sealed in a reducedpressure atmosphere such as vacuum or an inert gas atmosphere such asnitrogen or argon, for example.

As shown in FIG. 10, the container 70 has a container body 72 and a lid74. Of these, the container body 72 has a first recess 721 providedinside the container body 72, and a second recess 722 provided insidethe first recess 721 and deeper than the first recess 721. The containerbody 72 is made of, for example, a ceramic material, a resin material,or the like. Further, although not shown, the container body 72 includesan internal terminal provided on the inner surface, an external terminalprovided on the outer surface, wiring for coupling the internal terminaland the external terminal, and the like.

Further, the opening of the container body 72 is closed with the lid 74via a sealing member such as a seal ring or low melting point glass (notshown). As the constituent material of the lid 74, a material capable oftransmitting laser light, for example, a glass material or the like isused.

The optical modulation resonator 120 is arranged on the bottom surfaceof the first recess 721. The optical modulation resonator 120 issupported on the bottom surface of the first recess 721 by a joiningmember (not shown). Further, the internal terminal of the container body72 and the optical modulation resonator 120 are electrically coupled viaa conductive material (not shown) such as a bonding wire or a bondingmetal.

The circuit element 45 is arranged on the bottom surface of the secondrecess 722. The circuit element 45 is electrically coupled to theinternal terminal of the container body 72 via a bonding wire 76.Accordingly, the optical modulation resonator 120 and the circuitelement 45 are also electrically coupled via the wiring provided in thecontainer body 72. The circuit element 45 may be provided with a circuitother than the oscillation circuit 54 described later.

As described above, the optical modulator 12 according to the presentembodiment includes the container 70 which is a housing that houses thevibrator 30. The oscillation circuit 54 is also housed in the container70.

By adopting such a package structure, the optical modulation resonator120 and the circuit element 45 can be overlapped with each other, sothat the physical distance between the two can be reduced, and thewiring length between the optical modulation resonator 120 and thecircuit element 45 can be shortened. Therefore, it is possible tosuppress external noise from entering the drive signal S1 andconversely, the drive signal S1 from becoming a noise source. Further,in one container 70, both the optical modulation resonator 120 and thecircuit element 45 can be protected from the external environment.Therefore, the reliability of the laser interferometer 1 can be improvedwhile reducing the size of the sensor head section 51.

The structure of the container 70 is not limited to the structure shownin the drawing, and for example, the optical modulation resonator 120and the circuit element 45 may have separate package structures.Further, although not shown, other circuit elements constituting theoscillation circuit 54 may be housed in the container 70. The container70 may be provided as needed and may be omitted.

1.6. Optical Path Length Variable Section

The optical path length variable section 13 shown in FIG. 2 is arrangedon the optical path 22 and changes the optical path length of theoptical path 22 through which the object light L3 propagates. Theoptical path length variable section 13 shown in FIG. 2 includes amovable optical element 132 and a drive portion 138 that drives themovable optical element 132.

The movable optical element 132 shown in FIG. 2 includes a mirrors 134and 135 that change the direction of the optical path 22 and a prism 136that is driven by the drive portion 138. The drive portion 138reciprocates the prism 136 so as to move away from or approach themirrors 134 and 135. The mirror 134 changes the direction of the opticalpath 22 so that the object light L3 emitted from the object 14 to bemeasured is reflected toward the prism 136. The prism 136 changes thedirection of the optical path 22 so that the object light L3 isreflected toward the mirror 135. The mirror 135 changes the direction ofthe optical path 22 so that the object light L3 is reflected toward thequarter wave plate 6.

When the prism 136 is moved, the distance between the mirrors 134 and135 and the prism 136 increases or decreases. Accordingly, the opticalpath length of the optical path 22, that is, the optical distance of theoptical path 22 also increases or decreases. The optical path length inthe present specification refers to an optical distance.

The mirrors 134, 135 and prism 136 can be replaced by various opticalcomponents that can change the direction of the optical path 22,respectively.

Examples of the drive portion 138 include a MEMS actuator, a piezoactuator, an electromagnetic drive actuator, and the like. MEMS refersto Micro Electro Mechanical Systems.

Further, a movable optical system used in a Fourier Transform InfraredSpectrometer (FTIR) engine can also be used for the optical path lengthvariable section 13.

2. Control Section

The control section 57 controls the operation of the optical path lengthvariable section 13 and the operation of the demodulation circuit 52.

Specifically, the control section 57 controls the operation of theoptical path length variable section 13 according to the operationstatus of the demodulation circuit 52. The demodulation circuit 52operates in two operation modes, a maximum amplitude recording mode anda measurement mode, which will be described later. The control section57 switches the operation of the optical path length variable section 13according to the operation modes.

Although not shown, the hardware configuration of the control section 57includes, for example, a processor, a memory, an external interface, andthe like coupled to each other by an internal bus. When the processorreads and executes the program stored in the memory, various operationsof the control section 57 such as switching operation between themaximum amplitude recording mode and the measurement mode are realized.

3. Oscillation Circuit

As shown in FIG. 1, the oscillation circuit 54 outputs the drive signalS1 input to the optical modulator 12 of the optical system 50. Further,the oscillation circuit 54 outputs the reference signal S2 input to thedemodulation circuit 52.

The oscillation circuit 54 is not particularly limited as long as it isa circuit capable of oscillating the vibrator 30, and circuits havingvarious configurations are used. FIG. 11 is a circuit diagram showing aconfiguration of a one-stage inverter oscillation circuit as an exampleof a circuit configuration.

The oscillation circuit 54 shown in FIG. 11 includes the circuit element45, a feedback resistor Rf, a first limiting resistor R1, a secondlimiting resistor R2, a first capacitor Cg, a second capacitor Cd, and athird capacitor C3.

The circuit element 45 is an inverter IC. Terminals X1 and X2 of thecircuit element 45 are terminals coupled to the inverter, respectively.A terminal GND is coupled to the ground potential and a terminal Vcc iscoupled to the power potential. A terminal Y is a terminal foroscillation output.

The first capacitor Cg is coupled between the terminal X1 and the groundpotential. Further, between the terminal X2 and the ground potential,the first limiting resistor R1 and the second capacitor Cd coupled inseries with each other are coupled in this order from the terminal X2side. Further, one end of the feedback resistor Rf is coupled betweenthe terminal X1 and the first capacitor Cg, and the other end of thefeedback resistor Rf is coupled between the terminal X2 and the firstlimiting resistor R1.

Further, one end of the second limiting resistor R2 is coupled betweenthe first limiting resistor R1 and the second capacitor Cd. Further, theabove-mentioned vibrator 30 is coupled between the first capacitor Cgand the feedback resistor Rf and the other end of the second limitingresistor R2. That is, the vibrator 30 serves as a signal source of theoscillation circuit 54.

Further, FIG. 12 is an example of an LCR equivalent circuit of thevibrator 30.

As shown in FIG. 12, the LCR equivalent circuit of the vibrator 30 iscomposed of a series capacitance C₁, a series inductance L₁, anequivalent series resistance R₁, and a parallel capacitance C₀.

In the oscillation circuit 54 shown in FIG. 11, when the capacitance ofthe first capacitor Cg is C_(g) and the capacitance of the secondcapacitor Cd is C_(d), a load capacitance C_(L) is given by thefollowing equation (a).

$\begin{matrix}{C_{L} = \frac{C_{d}C_{g}}{C_{d} + C_{g}}} & (a)\end{matrix}$

Then, an oscillation frequency f_(osc) output from the terminal Y of theoscillation circuit 54 is given by the following equation (b).

$\begin{matrix}{f_{osc} = {f_{Q}\sqrt{1 + \frac{C_{1}}{C_{0} + C_{L}}}}} & (b)\end{matrix}$

f_(Q) is the natural frequency of the vibrator 30.

According to the above equation (b), it can be seen that the oscillationfrequency f_(osc) of the signal output from the terminal Y can be finelyadjusted by appropriately changing the load capacitance C_(L).

Further, a difference Δf between the natural frequency f_(Q) of thevibrator 30 and the oscillation frequency f_(osc) of the oscillationcircuit 54 is given by the following equation (c).

$\begin{matrix}{{\Delta f} = {{f_{osc} - f_{Q}} = {f_{Q}\left( {\sqrt{1 + \frac{C_{1}}{C_{0} + C_{L}}} - 1} \right)}}} & (c)\end{matrix}$

Here, since C₁<<C₀ and C₁<<C_(L), Δf is approximately given by thefollowing equation (d).

$\begin{matrix}{{\Delta f} = {{f_{osc} - f_{Q}} \cong {\frac{C_{1}}{2\left( {C_{0} + C_{L}} \right)}f_{Q}}}} & (d)\end{matrix}$

Therefore, the oscillation frequency f_(osc) of the oscillation circuit54 becomes a value corresponding to the natural frequency f_(Q) of thevibrator 30.

When the vibrator 30 is fixed to, for example, the container 70, thenatural frequency f_(Q) fluctuates when it receives expansion stress dueto temperature through the fixed portion. Further, when the vibrator 30is tilted, the natural frequency f_(Q) fluctuates due to the influenceof the gravity effect of its own weight.

In the oscillation circuit 54, even if the natural frequency f_(Q)fluctuates for this reason, the oscillation frequency f_(osc) changes inconjunction with the fluctuation based on the above equation (d). Thatis, the oscillation frequency f_(osc) is always a value deviated fromthe natural frequency f_(Q) by Δf. Accordingly, the vibrator 30 canstably obtain a displacement amplitude L₀. If the displacement amplitudeL₀ can be stabilized, the modulation characteristics of the opticalmodulator 12 can be stabilized, and the demodulation accuracy of thesample signal in the demodulation circuit 52 can be improved.

As an example, Δf=f_(osc)−f_(Q)≤600 [Hz] is preferable, and 240[Hz]≤Δf≤450 [Hz] is more preferable.

Instead of the oscillation circuit 54, a signal generator such as afunction generator or a signal generator may be used.

4. Demodulation Circuit

The demodulation circuit 52 performs a demodulation process fordemodulating a sample signal derived from the object 14 to be measuredfrom the light reception signal output from the photodetector 10. Thesample signal is, for example, a phase signal or a frequency signal.Displacement information of the object 14 to be measured can be acquiredfrom the phase signal. Further, velocity information of the object 14 tobe measured can be acquired from the frequency signal. If differentinformation can be acquired in this way, the function as a displacementmeter or a velocimeter can be provided, so that the laser interferometer1 can be enhanced in functionality.

The circuit configuration of the demodulation circuit 52 is setaccording to the modulation processing method. In the laserinterferometer 1 according to the present embodiment, the opticalmodulator 12 including the vibrator 30 is used. Since the vibrator 30 isa simple vibrator, the vibration velocity changes from moment to moment.Therefore, the modulation frequency also changes, and the demodulationcircuit in the related art cannot be used as it is.

The demodulation circuit in the related art refers to a circuit thatdemodulates a sample signal from a light reception signal including amodulated signal modulated by using an acousto-optic modulator (AOM). Inthe acousto-optic modulator, the modulation frequency does not change.Therefore, the demodulation circuit in the related art can demodulatethe sample signal from the light reception signal including a modulatedsignal modulated by the optical modulator whose modulation frequencydoes not change. However, when a modulated signal modulated by theoptical modulator 12 whose modulation frequency changes is included, themodulated signal cannot be demodulated as it is.

Therefore, the demodulation circuit 52 shown in FIG. 1 includes apreprocessing section 53 and a demodulation section 55. The lightreception signal output from the photodetector 10 is first passedthrough the preprocessing section 53 and then guided to the demodulationsection 55. The preprocessing section 53 preprocesses the lightreception signal. By this preprocessing, a signal that can bedemodulated by the demodulation circuit in the related art can beobtained. Therefore, in the demodulation section 55, the sample signalderived from the object 14 to be measured is demodulated by a knowndemodulation method.

4.1. Configuration of Preprocessing Section

The preprocessing section 53 shown in FIG. 1 includes a first bandpassfilter 534, a second bandpass filter 535, a first delay adjuster 536, asecond delay adjuster 537, a multiplier 538, a third bandpass filter539, a first recording section 540, a second recording section 541, afirst AGC 542, a second AGC 543, a third AGC 544, a fourth AGC 545, anda summer 546. AGC stands for an Auto Gain Control.

Further, a current-voltage converter 531 and an ADC 532 are coupled inthis order from the photodetector 10 side between the preprocessingsection 53 and the photodetector 10.

Further, an ADC 533 is coupled between the oscillation circuit 54 andthe second delay adjuster 537.

The current-voltage converter 531 is a transimpedance amplifier, whichconverts a current output from the photodetector 10 into a voltagesignal. The ADCs 532 and 533 are analog-to-digital converters, whichconvert an analog signal into a digital signal with a predeterminednumber of sampling bits.

The first bandpass filter 534, the second bandpass filter 535, and thethird bandpass filter 539 are filters that selectively transmit signalsin a specific frequency band.

The first delay adjuster 536 and the second delay adjuster 537 arecircuits that adjust the signal delay. The multiplier 538 is a circuitthat generates an output signal proportional to the product of two inputsignals.

The first recording section 540 is a detection/recording circuit thatdetects the maximum amplitude of the signal output from the first delayadjuster 536 and records it as a first recording maximum amplitude. Thesecond recording section 541 is a detection/recording circuit thatdetects the maximum amplitude of the signal output from the thirdbandpass filter 539 and records it as a second recording maximumamplitude.

The first AGC 542 adjusts the amplitude of the signal passing throughthe first recording section 540 based on the first recording maximumamplitude recorded in the first recording section 540. The second AGC543 adjusts the amplitude of the signal passing through the secondrecording section 541 based on the second recording maximum amplituderecorded in the second recording section 541.

The third AGC 544 and the fourth AGC 545 are circuits that align theamplitudes of signals with each other. The summer 546 is a circuit thatgenerates an output signal proportional to the sum of two input signals.

The current output output from the photodetector 10 is converted into avoltage signal by the current-voltage converter 531. The voltage signalis converted into a digital signal by the ADC 532 and divided into afirst signal and a second signal. In FIG. 1, the path of the firstsignal is referred to as a first signal path ps1, and the path of thesecond signal is referred to as a second signal path ps2.

After the first signal is passed through the first bandpass filter 534arranged on the first signal path ps1, a group delay is adjusted by thefirst delay adjuster 536. The group delay adjusted by the first delayadjuster 536 corresponds to a group delay of the second signal by thesecond bandpass filter 535, which will be described later. Due to thisdelay adjustment, the delay time associated with the passage of thefilter circuit can be made uniform between the first bandpass filter 534through which the first signal passes and the second bandpass filter 535and the third bandpass filter 539 through which the second signalpasses. The first signal that has passed through the first delayadjuster 536 is input to the first recording section 540. The firstrecording section 540 detects the maximum amplitude of the first signalin the maximum amplitude recording mode described later, and records itas the first recording maximum amplitude. Then, in the measurement modedescribed later, the first recording section 540 calculates a correctioncoefficient M₁ based on the first recording maximum amplitude recordedin the maximum amplitude recording mode and the maximum amplitude of thefirst signal newly detected in the measurement mode. The first AGC 542adjusts the amplitude of the first signal passing through the firstrecording section 540 based on the correction coefficient M₁ calculatedby the first recording section 540. After that, the first signal isinput to the summer 546 via the third AGC 544.

The second signal is passed through the second bandpass filter 535arranged on the second signal path ps2 and then input to the multiplier538. In the multiplier 538, a reference signal cos(ω_(m)t) output fromthe second delay adjuster 537 is multiplied by the second signal.Specifically, the reference signal S2 output from the oscillationcircuit 54 is digitally converted by the ADC 533, the phase is adjustedby the second delay adjuster 537, and the converted signal is output tothe multiplier 538. After that, the second signal is passed through thethird bandpass filter 539 and then input to the second recording section541. The second recording section 541 detects the maximum amplitude ofthe second signal in the maximum amplitude recording mode describedlater, and records it as the second recording maximum amplitude. Then,in the measurement mode described later, the second recording section541 calculates a correction coefficient M₂ based on the second recordingmaximum amplitude recorded in the maximum amplitude recording mode andthe maximum amplitude of the second signal newly detected in themeasurement mode. The second AGC 543 adjusts the amplitude of the secondsignal passing through the second recording section 541 based on thecorrection coefficient M₂ calculated by the second recording section541. After that, the second signal is input to the summer 546 via thefourth AGC 545.

In the summer 546, an output signal proportional to the sum of the firstsignal and the second signal is output to the demodulation section 55.

4.2. Basic Principle of Preprocessing by Preprocessing Section

Next, the basic principle of preprocessing in the preprocessing section53 will be described. The basic principle mentioned here refers to theprinciple of preprocessing in which the first recording section 540, thesecond recording section 541, the first AGC 542, and the second AGC 543do not contribute, which is the principle described in JP-A-2-38889.Here, when E_(m), E_(d), and φ are defined as

E _(m) =a _(m){cos(ω₀ t+B sin ω_(m) t+ϕ _(m))+i sin(ω₀ t+B sin ω_(m) t+ϕ_(m))}  (1)

E _(d) =a _(d){cos(ω₀ t+A sin ω_(d) t+ϕ _(d))+i sin(ω₀ t+A sin ω_(d) t+ϕ_(d))}  (2)

ϕ=ϕ_(m)ϕ_(d)   (3)

a light reception signal intensity I_(PD) output from the photodetector10 is theoretically represented by the following equation.

$\begin{matrix}\begin{matrix}{I_{PD} = \left\langle {{E_{m} + E_{d}}}^{2} \right\rangle} \\{= \left\langle {{E_{m}^{2} + E_{d}^{2} + {2E_{m}E_{d}}}} \right\rangle} \\{= {a_{m}^{2} + a_{d}^{2} + {2a_{m}{a_{d}\left( {{B\;\sin\;\omega_{m}t} - {A\;\sin\;\omega_{d}t} + \phi} \right)}}}}\end{matrix} & (4)\end{matrix}$

E_(m), E_(d), φ_(m), φ_(d), φ, ω_(m), ω_(d), ω₀, a_(m), and a_(d) are asfollows.

-   E_(m): Electric field component of modulated signal-   E_(d): Electric field component of sample signal derived from object    to be measured-   φ_(m): Initial phase value of optical path 20-   φ_(d): Initial phase value of optical path 22-   φ: Optical path phase difference of laser interferometer-   ω_(m): Angular frequency of modulated signal derived from optical    modulator-   ω_(d): Angular frequency of sample signal derived from object to be    measured-   ω₀: Angular frequency of emission light emitted from light source    section-   a_(m): Coefficient-   a_(d): Coefficient

Further, < > in the equation (4) represents a time average.

The first and second terms of the above equation (4) represents a DCcomponent, and the third term represents an AC component. When this ACcomponent is referred to as I_(PD.AC), I_(PD.AC) is as follows.

$\begin{matrix}\begin{matrix}{I_{PDAC} = {2a_{m}{a_{d}\left( {{B\;\sin\;\omega_{m}t} - {A\;\sin\;\omega_{d}t} + \phi} \right)}}} \\{= {2a_{m}{a_{d}\left\lbrack {{{\cos\left( {B\;\sin\;\omega_{m}t} \right)}{\cos\left( {{A\;\sin\;\omega_{d}t} - \phi} \right)}} +} \right.}}} \\\left. {\sin\left( {B\;\sin\;\omega_{m}t} \right){\sin\left( {{A\;\sin\;\omega_{d}t} - \phi} \right)}} \right\rbrack\end{matrix} & (5) \\{A = \frac{d_{dmax}}{f_{d}}} & (6) \\{B = \frac{f_{mmax}}{f_{m}}} & (7) \\{{f_{dmax}\text{:}\mspace{14mu}{Amplitude}\mspace{14mu}{of}\mspace{14mu}{sample}\mspace{14mu}{signal}}{f_{d}\text{:~~~}{Frequency}\mspace{14mu}{of}\mspace{14mu}{sample}\mspace{14mu}{signal}}{f_{mmax}\text{:~~~}{Amplitude}\mspace{14mu}{of}\mspace{14mu}{modulated}\mspace{14mu}{signal}}{f_{m}\text{:}\mspace{14mu}{Frequency}\mspace{14mu}{of}\mspace{14mu}{modulated}\mspace{14mu}{signal}}} & \;\end{matrix}$

Here, a υ-order Bessel function as shown in the following equation isknown.

cos{ζ sin(2πf _(v) t)}=J ₀(ζ)+2J ₂(ζ)cos(2·2πf _(v) t)+2J ₄(ζ)cos(4·2πf_(v) t)+  (8)

sin{ζ sin(2πf _(v) t)}=2J ₁(ζ)sin(1·2πf _(v) t)+2J ₃(ζ)sin(3·2πf _(v)t)+  (9)

When the above equation (5) is series-expanded using the Besselfunctions of the above equations (8) and (9), it can be transformed asfollows.

I _(PD.AC)=2a _(m) a _(d)[{J ₀(B)+2J ₂(B)cos(2·ω_(m) t)+2J₄(B)cos(4·ω_(m) t)+ . . . }cos(A sin ω_(d) t−ϕ)−{2J ₁(B)sin(1·ω_(m)t)+2J ₃(B)sin(3·ω_(m) t)+ . . . }sin(A sin ω_(d) t−ϕ)]  (10)

However, J₀(B), J₁(B), J₂(B), . . . are Bessel coefficients,respectively.

When expanded as described above, it can be said that theoretically, theband corresponding to a specific order can be extracted by a bandpassfilter.

Therefore, the preprocessing section 53 described above preprocesses thelight reception signal in the following flow based on this theory.

First, the digital signal output from the above-mentioned ADC 532 isdivided into two, a first signal and a second signal. The first signalis passed through the first bandpass filter 534. The central angularfrequency of the first bandpass filter 534 is set to ω_(m). Accordingly,the first signal after passing through the first bandpass filter 534 isrepresented by the following equation.

I _(pass1) =J ₁(B){−cos(ω_(m) t+A sin ω_(d) t−ϕ)+cos(ω_(m) t−A sin ω_(d)t+ϕ)}=−2J ₁(B)sin(ω_(m) t)sin(A sin ω_(d) t−ϕ)   (11)

On the other hand, the second signal is passed through the secondbandpass filter 535. The central angular frequency of the secondbandpass filter 535 is set to a value different from the central angularfrequency of the first bandpass filter 534. Here, as an example, thecentral angular frequency of the second bandpass filter 535 is set to2ω_(m). Accordingly, the second signal after passing through the secondbandpass filter 535 is represented by the following equation.

$\begin{matrix}\begin{matrix}{I_{BPF2} = {{J_{2}(B)}{{\cos\left( {{2 \cdot \omega_{m}}t} \right)} \cdot {\cos\left( {{A\;\sin\;\omega_{d}t} - \phi} \right)}}}} \\{= {\frac{1}{2}{J_{2}(B)}\left\{ {{\cos\left( {{{2 \cdot \omega_{m}}t} + {\cos\left( {{A\sin\omega_{d}t} - \phi} \right)}} \right)} +} \right.}} \\\left. {\cos\left( {{{2 \cdot \omega_{m}}t} - {\cos\left( {{A\sin\omega_{d}t} - \phi} \right)}} \right)} \right\}\end{matrix} & (12)\end{matrix}$

The second signal after passing through the second bandpass filter 535is multiplied by the reference signal cos(ω_(m)t) by the multiplier 538.The second signal after multiplication is represented by the followingequation.

$\begin{matrix}\begin{matrix}{I_{\cos{({\omega_{m}t})}} = {\frac{1}{2}{J_{2}(B)}\left\{ {{\cos\left( {{{2 \cdot \omega_{m}}t} + {A\;\sin\;\omega_{d}t} - \phi} \right)} +} \right.}} \\{{\cos\left( {{{2 \cdot \omega_{m}}t} - {A\;\sin\;\omega_{d}t} + \phi} \right)} \cdot {\cos\left( {\omega_{m}t} \right)}} \\{= {\frac{1}{2}{J_{2}(B)}\left\{ {{\cos\left( {{{3 \cdot \omega_{m}}t} + {A\;\sin\;\omega_{d}t} - \phi} \right)} +} \right.}} \\{{\cos\left( {{{1 \cdot \omega_{m}}t} + {A\;\sin\;\omega_{d}t} - \phi} \right)} + {\cos\left( {{{2 \cdot \omega_{m}}t} -} \right.}} \\{\left. {{A\;\sin\;\omega_{d}t} + \phi} \right) \cdot {\cos\left( {\omega_{m}t} \right)}}\end{matrix} & (13)\end{matrix}$

The second signal after passing through the multiplier 538 is passedthrough the third bandpass filter 539. The central angular frequency ofthe third bandpass filter 539 is set to the same value as the centralangular frequency of the first bandpass filter 534. Here, as an example,the central angular frequency of the third bandpass filter 539 is set toω_(m). Accordingly, the second signal after passing through the thirdbandpass filter 539 is represented by the following equation.

$\begin{matrix}\begin{matrix}{I_{{pass}\; 2} = {\frac{1}{2}{J_{2}(B)}\left\{ {{\cos\left( {{\omega_{m}t} + {A\sin\omega_{d}t} - \phi} \right)} + {\cos\left( {{\omega_{m}t} - {A\;\sin\;\omega_{d}t} + \phi} \right)}} \right\}}} \\{= {{J_{2}(B)}{\cos\left( {\omega_{m}t} \right)}{\cos\left( {{A\;\sin\;\omega_{d}t} - \phi} \right)}}}\end{matrix} & (14)\end{matrix}$

After that, the phase and amplitude of the first signal represented bythe above equation (11) are adjusted with the first delay adjuster 536and the third AGC 544. In the third AGC 544, 1/(−2J₁(B)) is multiplied.

Further, the amplitude of the second signal represented by the aboveequation (14) is adjusted by the fourth AGC 545. In the fourth AGC 545,1/J₂(B) is multiplied.

Then, the first signal and the second signal are summed by the summer546. The summing result is represented by the following equation.

I ₅₃=cos(ω_(m) t+A sin ω_(d) t−ϕ)   (15)

As a result of the summation, the unnecessary term disappears and thenecessary term can be taken out as in the above equation (15). Thisresult is output to the demodulation section 55.

4.3. Configuration of Demodulation Section

The demodulation section 55 performs a demodulation process fordemodulating a sample signal derived from the object 14 to be measuredfrom the signal output from the preprocessing section 53. Thedemodulation process is not particularly limited, and examples thereofinclude a known orthogonal detection method. The orthogonal detectionmethod is a method of performing the demodulation process by performingan operation of mixing signals orthogonal to each other from the outsidewith respect to an input signal.

The demodulation section 55 shown in FIG. 1 is a digital circuitincluding a first multiplier 551, a second multiplier 552, a phaseshifter 553, a first low-pass filter 555, a second low-pass filter 556,a divider 557, an inverse tangent calculator 558, and a signal outputcircuit 559.

4.4. Principle of Demodulation Process by Demodulation Section

In the demodulation process, first, the signal output from thepreprocessing section 53 is divided into two. The first multiplier 551multiplies one of the divided signals by a frequency signal cos(ω_(m)t),which is the reference signal S2 output from the oscillation circuit 54.The second multiplier 552 multiplies the other of the divided signals bya frequency signal −sin(ω_(m)t) obtained by shifting the phase of thereference signal S2 output from the oscillation circuit 54 by −90° bythe phase shifter 553. The frequency signal cos(ω_(m)t) and thefrequency signal −sin(ω_(m)t) are signals that are 90° out of phase witheach other.

The signal passed through the first multiplier 551 is passed through thefirst low-pass filter 555 and then input to the divider 557 as a signalx. The signal passed through the second multiplier 552 is also passedthrough the second low-pass filter 556, and then input to the divider557 as a signal y. The divider 557 divides the signal y by the signal x,passes the signal y/x through the inverse tangent calculator 558, andobtains a signal at an(y/x).

After that, by passing the signal a tan(y/x) through the signal outputcircuit 559, a phase φ_(d) is obtained as the sample signal derived fromthe object 14 to be measured. Then, the displacement information of theobject 14 to be measured can be calculated based on the phase φ_(d).Thereby, a displacement meter that measures the displacement of theobject 14 to be measured is realized. In addition, velocity informationcan be obtained from the displacement information. Thereby, avelocimeter that measures the velocity of the object 14 to be measuredis realized.

Although the circuit configuration of the demodulation section 55 hasbeen described above, the circuit configuration of the above digitalcircuit is an example and is not limited thereto. Further, thedemodulation section 55 is not limited to a digital circuit, and may bean analog circuit. The analog circuit may include an F/V convertercircuit and a ΔΣ counter circuit.

Further, in the circuit configuration of the demodulation section 55described above, a frequency signal may be obtained as a sample signalderived from the object 14 to be measured. The velocity information ofthe object 14 to be measured can be calculated based on the frequencysignal.

4.5. Demodulatorable Condition

Here, in the basic principle of preprocessing in the preprocessingsection 53 described above, the amplitude of the first signal and theamplitude of the second signal are aligned with each other in the thirdAGC 544 and the fourth AGC 545. That is, it is necessary to align thecoefficient −2J₁(B) included in the equation (11) and the coefficientJ₂(B) included in the equation (14). J₁(B) and J₂(B) included in thesecoefficients are the Bessel coefficients described above, and B amongthem is the ratio of the amplitude of the modulated signal to thefrequency as described above. On the other hand, A included in theequations (11) and (14) is the ratio of the amplitude of the samplesignal to the frequency, as described above. B is a value that isdetermined by the setting of the optical system 50 and takes a knownconstant value. For this reason, the amplitudes can be adjusted in thethird AGC 544 and the fourth AGC 545.

However, such an amplitude adjustment is possible only when a portionother than the coefficient −2J₁(B) in the equation (11) and a portionother than the coefficient J₂(B) in the equation (14) periodicallyfluctuate within the range of the maximum value 1 and the minimum value−1, respectively. The portion other than the coefficient −2J₁(B) in theequation (11) is represented by the following equation (16). The portionother than the coefficient J₂(B) in the equation (14) is represented bythe following equation (17).

I _(pass1′)=sin(ω_(m) t)sin(A sin ω_(d) t−ϕ)   (16)

I _(pass2′)=cos(ω_(m) t)cos(A sin ω_(d) t−ϕ)   (17)

Therefore, in order to enable the adjustment of the amplitudes in thethird AGC 544 and the fourth AGC 545, it is necessary to satisfy thecondition that the equations (16) and (17) periodically fluctuate withinthe range of the maximum value 1 and the minimum value −1, respectively.

Here, the angular frequency ω_(m) of the modulated signal issufficiently larger than the angular frequency ω_(d) of the samplesignal, and ω_(m)>>ω_(d) holds. Therefore, in order to satisfy the aboveconditions, it is a condition that the maximum value of the absolutevalue of sin(A sin ω_(d)t−φ) in the equation (16) is 1, and the maximumvalue of the absolute value of cos(A sin ω_(d)t−φ) in the equation (17)is 1. When this condition is satisfied, preprocessing by thepreprocessing section 53 becomes possible, and finally demodulationprocess by the demodulation section 55 becomes possible. Therefore, thiscondition is called a “demodulatorable condition”.

Summarizing the above, it is a demodulatorable condition that both thefollowing equations (18) and (19) hold.

max{|sin(A sin ω_(d) t−ϕ)|}=1   (18)

max{|cos(A sin ω_(d) t−ϕ)|}=1   (19)

In order to establish this demodulatorable condition, the range ofvalues of A sin ω_(d)t−φ should be π or more. Here, the optical pathphase difference φ cannot usually take any value. Then, in order for therange of values of A sin ω_(d)t−φ to be π or more, it is required tosatisfy the following equation, especially for A.

$\begin{matrix}{A = {\frac{f_{d\max}}{f_{d}} \geq \frac{\pi}{2}}} & (20)\end{matrix}$

On the other hand, when the equation (20) does not hold, it means thatthe demodulatorable condition cannot be satisfied. Therefore, in thepresent embodiment, the optical system 50 is provided with a mechanismfor changing the optical path phase difference φ so that thedemodulatorable condition can be satisfied even when the equation (20)does not hold. Hereinafter, a method of satisfying the demodulatorablecondition using this mechanism will be described.

4.6. Principle of Establishing Demodulatorable Condition

Here, in the laser interferometer 1 according to the present embodiment,the principle of establishing the above demodulatorable condition evenwhen the equation (20) does not hold will be described.

Since A is a value that depends on the object 14 to be measured, Acannot be adjusted to a desired value when the optical path phasedifference φ of the optical system 50 is fixed. Therefore, the opticalsystem 50 shown in FIG. 2 is provided with the optical path lengthvariable section 13. As described above, the optical path lengthvariable section 13 is arranged on the optical path 22 and changes theoptical path length of the optical path 22 through which the objectlight L3 propagates. When the optical path length of the optical path 22changes, the optical path phase difference φ included in the aboveequations (11) and (14) becomes a function φ(t) of time. Therefore, theequations (11) and (14) are as follows.

I _(pass1)=−2J ₁(B)sin(ω_(m) t)sin(A sin ω_(d) t−ϕ(t))   (11-2)

I _(pass2) =J ₂(B)cos(ω_(m) t)cos(A sin ω_(d) t−ϕ(t))   (14-2)

In order for the above equations (11-2) and (14-2) to hold, it isdesirable that, in the optical path length variable section 13, theoptical path length reciprocates (vibrates) relatively slowly, that is,the frequency of vibration of the optical path length variable section13 is sufficiently smaller than the frequency f_(d) of the samplesignal. Specifically, it is preferably about 10 Hz to 1 kHz.

When the movable optical element 132 of the optical path length variablesection 13 is reciprocated at such a frequency, the optical path phasedifference φ vibrates at that frequency. Then, even if the aboveequation (20) does not hold for A, the time when the demodulatorablecondition is satisfied appears instantaneously.

Therefore, in the present embodiment, the amplitude of the first signalrepresented by the equation (11-2) is monitored in the first recordingsection 540 of FIG. 1. Then, when the maximum amplitude is reached, thevalue is recorded in the first recording section 540 as the firstrecording maximum amplitude. Further, the second recording section 541of FIG. 1 monitors the amplitude of the second signal represented by theequation (14-2). Then, when the maximum amplitude is reached, the valueis recorded in the second recording section 541 as the second recordingmaximum amplitude.

When the time when the first recording maximum amplitude is detected ist_(p1) and the time when the second recording maximum amplitude isdetected is t_(p2), the above-mentioned demodulatorable condition isexpressed as follows, and is satisfied at this moment.

max{|sin(A sin ω_(d) t−ϕ(t=t _(p1)))|}=1   (18-2)

max{|cos(A sin ω_(d) t−ϕ(t=t _(p2)))|}=1   (19-2)

The first recording maximum amplitude and the second recording maximumamplitude recorded as described above are temporarily recorded in thefirst recording section 540 and the second recording section 541. Then,when the sample signal derived from the object 14 to be measured isacquired, the recorded value is used to correct the first signal and thesecond signal. Thereby, even if the above equation (20) does not hold,the demodulation process in the demodulation circuit 52 becomespossible.

The laser interferometer 1 operates according to the above principle.Hereinafter, the operation mode of the laser interferometer 1 will bedescribed.

4.7. Operation Mode of Laser Interferometer

As described above, the operation mode of the laser interferometer 1includes a maximum amplitude recording mode and a measurement mode.Hereinafter, these will be described in order.

4.7.1. Maximum Amplitude Recording Mode

The control section 57 of the laser interferometer 1 controls theoptical path length variable section 13 to operate when the maximumamplitude recording mode is selected. Then, the emission light L1 isemitted to the object 14 to be measured and the optical modulator 12 ina state where the optical path length variable section 13 is operated,that is, in a state where the movable optical element 132 is vibrated.Then, the optical path length of the optical path 22 through which theobject light L3 propagates changes, and the amplitudes of the firstsignal and the second signal obtained from the light reception signalalso change.

When the amplitude of the first signal reaches the maximum value at timet_(p1), the first recording section 540 records the value as a firstrecording maximum amplitude I_(Rmax1). When the amplitude of the secondsignal reaches the maximum value at time t_(p2), the second recordingsection 541 records the value as a second recording maximum amplitudeI_(Rmax2). The first recording maximum amplitude I_(Rmax1) and thesecond recording maximum amplitude I_(Rmax2) are expressed as follows.

I _(Rmax1)=max{−2J ₁(B)sin(ω_(m) t)sin(A sin ω_(d) t−ϕ(t=t_(p1)))}  (11-3)

I _(Rmax2)=max{J ₂(B)cos(ω_(m) t)cos(A sin ω_(d) t−ϕ(t=t_(p2)))}  (14-3)

When the optical path length is changed by the optical path lengthvariable section 13, the optical path length may be changed by anoptical distance or more, where the optical distance corresponds to thewavelength of the emission light L1 (first laser light). Accordingly,the first recording maximum amplitude I_(Rmax1) and the second recordingmaximum amplitude I_(Rmax2) that satisfy the demodulatorable conditioncan be reliably recorded.

Specifically, the optical path length of the optical path 20 in FIG. 2is L₀₁, and the optical path length of the optical path 22 is L₀₂. Then,when the difference between the optical distance of the path throughwhich the reference light L2 propagates and the optical distance of thepath through which the object light L3 propagates is L, L=2(L₀₁−L₀₂).

On the other hand, when n is the refractive index of the medium and λ isthe wavelength of the emission light L1, the initial phase is 2πnL/λ.

If L is L₀ when the initial phase of the laser interferometer 1 is 0 andL is L_(2π) when the initial phase is 2π, 2πn(L_(2π)−L₀)/λ=2π andn(L_(2π)−L₀)=λ. Therefore, the optical path length n(L_(2π)−L₀) to bechanged by the optical path length variable section 13 may be equal toor more than the optical distance of the wavelength λ of the emissionlight L1.

4.7.2. Measurement Mode

When the measurement mode is selected, the control section 57 stops theoperation of the optical path length variable section 13. In this state,the optical path phase difference φ is constant. Then, in this state,the emission light L1 is emitted to the object 14 to be measured and theoptical modulator 12. In addition, the control section 57 controls theoperation of the first recording section 540 to detect and record amaximum amplitude I_(Mmax1) of the first signal obtained from the lightreception signal. Further, the control section 57 controls the operationof the second recording section 541 to detect and record a maximumamplitude I_(Mmax2) of the second signal obtained from the lightreception signal. The maximum amplitude I_(Mmax1) of the first signaland the maximum amplitude I_(Mmax2) of the second signal in themeasurement mode are expressed as follows.

I _(Mmax1)=max{−2J ₁(B)sin(ω_(m) t)sin(A sin ω_(d) t−ϕ)}  (11-4)

I _(Mmax2)=max{J ₂(B)cos(ω_(m) t)cos(A sin ω_(d) t−ϕ)}  (14-4)

At this time, the first recording section 540 calculates the ratio ofthe first recording maximum amplitude I_(Rmax1) to the maximum amplitudeI_(Mmax1) as the correction coefficient M₁. Further, the secondrecording section 541 calculates the ratio of the second recordingmaximum amplitude I_(Rmax2) to the maximum amplitude I_(Mmax2) as thecorrection coefficient M₂. The correction coefficient M₁ and thecorrection coefficient M₂ are expressed as follows.

$\begin{matrix}{\frac{I_{{Rmax}\; 1}}{I_{M\max 1}} = {\frac{\max\left\{ {{- 2}{J_{1}(B)}{\sin\left( {\omega_{m}t} \right)}{\sin\left( {{A\sin\omega_{d}t} - {\phi\left( {t = t_{p\; 1}} \right)}} \right)}} \right\}}{\max\left\{ {{- 2}{J_{1}(B)}{\sin\left( {\omega_{m}t} \right)}{\sin\left( {{A\sin\omega_{d}t} - \phi} \right)}} \right\}} = M_{1}}} & (21) \\{\frac{I_{{Rmax}\; 2}}{I_{{Mmax}\; 2}} = {\frac{\max\left\{ {{J_{2}(B)}{{\cos\left( {\omega_{m}t} \right)} \cdot {\cos\left( {{A\;\sin\;\omega_{d}t} - {\phi\left( {t = t_{p2}} \right)}} \right)}}} \right\}}{\max\left\{ {{J_{2}(B)}{{\cos\left( {\omega_{m}t} \right)} \cdot {\cos\left( {{A\;\sin\;\omega_{d}t} - \phi} \right)}}} \right\}} = M_{2}}} & (22)\end{matrix}$

Such a correction coefficient M₁ is multiplied by the first signal inthe first AGC 542. Further, the correction coefficient M₂ is multipliedby the second signal in the second AGC 543. As a result, the firstsignal that has passed through the first AGC 542 and the second signalthat has passed through the second AGC 543 are expressed as follows.

I _(pass1-AGC1)=−2J ₁(B)sin(ω_(m) t)M ₁ sin(A sin ω_(d) t−ϕ)   (11-5)

I _(pass2-AGC2) =J ₂(B)cos(ω_(m) t)M ₂ cos(A sin ω_(d) t−ϕ)   (14-5)

When the above correction is performed, the maximum amplitude of thefirst signal and the maximum amplitude of the second signal can bealigned with each other in the third AGC 544 and the fourth AGC 545 asdescribed in the above-mentioned basic principle. As described above, inthe third AGC 544, the first signal is multiplied by 1/(−2J₁(B)).Further, in the fourth AGC 545, the second signal is multiplied by1/J₂(B). As a result, the first signal that has passed through the thirdAGC 544 and the second signal that has passed through the fourth AGC 545are expressed as follows.

I _(pass1-AGC3)=sin(ω_(m) t)M ₁ sin(A sin ω_(d) t−ϕ)   (11-6)

I _(pass2-AGC4)=cos(ω_(m) t)M ₂ cos(A sin ω_(d) t−ϕ)   (14-6)

As a result of aligning the maximum amplitudes as described above, bysumming the first signal and the second signal in the summer 546described above, unnecessary terms can be eliminated and necessary termscan be extracted as described above. As a result, even when the aboveequation (20) does not hold, the sample signal can be demodulated, andthe displacement information and the velocity information can bemeasured.

In the above calculation process, the correction coefficients M₁ and M₂are calculated, and the calculation of multiplying the first signal andthe second signal is performed. Along with the calculation, there is aneffect of canceling a device error given to the signal amplitude by theoptical system 50, the demodulation circuit 52, or the like. Therefore,the present embodiment is also useful from the viewpoint that theinfluence of the device error on the measurement result can be reduced.

As described above, the laser interferometer 1 according to the presentembodiment includes the light source 2, the optical modulator 12, thephotodetector 10, and the optical path length variable section 13. Thelight source 2 emits the emission light L1 (first laser light). Theoptical modulator 12 includes the vibrator 30, and modulates theemission light L1 using the vibrator 30 to generate the reference lightL2 (second laser light) including a modulated signal. The photodetector10 receives interference light between the object light L3 (third laserlight) including a sample signal, which is derived from the object 14 tobe measured (object), generated by reflecting the emission light L1 onthe object 14 to be measured and the reference light L2 to output alight reception signal. The optical path length variable section 13changes the optical path length of the optical path 22 through which theobject light L3 propagates.

According to such a configuration, even if the element depending on theobject 14 to be measured, specifically the value of A described above,is an inappropriate value in the demodulation process in thedemodulation circuit 52, the light reception signal can be corrected soas to satisfy the demodulatorable condition. Accordingly, the laserinterferometer 1 capable of demodulating the sample signal in a widerband and calculating displacement information, velocity information, orthe like can be realized. Further, since the volume of the vibrator 30is very small and the electric power required for oscillation is alsosmall, it is easy to reduce the size and power consumption of the laserinterferometer 1.

Further, the laser interferometer 1 according to the present embodimentincludes the polarization beam splitter 4 (optical path branchingelement) that branches the optical path 18 through which the emissionlight L1 (first laser light) propagates. The optical path lengthvariable section 13 shown in FIG. 2 is arranged on the optical path 22between the polarization beam splitter 4 and the object 14 to bemeasured (object).

According to such a configuration, since the optical path lengthvariable section 13 can be arranged between the polarization beamsplitter 4 and the object 14 to be measured, although it depends on theconfiguration of the optical system 50, a laser interferometer 1 usefulfrom the viewpoint of ease of manufacture and maintainability for theoptical path length variable section 13 can be obtained.

Further, the optical path length variable section 13 according to thepresent embodiment includes the movable optical element 132 that changesthe optical path length by moving, and the drive portion 138 that drivesthe movable optical element 132.

According to such a configuration, the optical path length can bechanged with high accuracy, and the change can be made in nm units.Therefore, the demodulation accuracy in the demodulation circuit 52 canbe improved.

Further, the laser interferometer 1 according to the present embodimentincludes the demodulation circuit 52. As described above, thedemodulation circuit 52 includes a first signal path psi and a secondsignal path ps2 through which the light reception signal is passed.Then, the demodulation circuit 52 demodulates the sample signal from thefirst signal, which is alight reception signal that has passed throughthe first signal path ps1, and the second signal, which is a lightreception signal that has passed through the second signal path ps2.

Further, the demodulation circuit 52 has the first recording section540, the second recording section 541, the first AGC 542 (firstamplitude adjusting section), and the second AGC 543 (second amplitudeadjusting section).

The first recording section 540 detects the maximum amplitude of thefirst signal, which is a light reception signal passing through thefirst signal path ps1, and records it as the first recording maximumamplitude, in the maximum amplitude recording mode in which the opticalpath length is changed by the optical path length variable section 13.The second recording section 541 detects the maximum amplitude of thesecond signal, which is a light reception signal passing through thesecond signal path ps2, and records it as the second recording maximumamplitude, in the maximum amplitude recording mode in which the opticalpath length is changed by the optical path length variable section 13.

The first AGC 542 adjusts the maximum amplitude of the first signalpassing through the first signal path ps1 in the measurement mode basedon the first recording maximum amplitude. The second AGC 543 adjusts themaximum amplitude of the second signal passing through the second signalpath ps2 in the measurement mode based on the second recording maximumamplitude.

According to such a configuration, even if the value of A describedabove is an inappropriate value in the demodulation process in thedemodulation circuit 52, the light reception signal can be corrected soas to satisfy the demodulatorable condition with a relatively simpleconfiguration. Further, when performing an operation for correcting themaximum amplitude of the signal detected in the measurement mode basedon the maximum amplitude recorded in the maximum amplitude recordingmode, since the device error given to the signal amplitude by theoptical system 50 or the like can be canceled, the influence of thedevice error on the demodulation accuracy can be reduced.

5. Modification Example

Next, laser interferometers according to modification examples will bedescribed.

FIG. 13 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a first modification example. FIG. 14 is a schematicconfiguration diagram showing a mounting structure of an optical systemincluded in a laser interferometer according to a second modificationexample. FIG. 15 is a schematic configuration diagram showing a mountingstructure of an optical system included in a laser interferometeraccording to a third modification example. FIG. 16 is a schematicconfiguration diagram showing amounting structure of an optical systemincluded in a laser interferometer according to a fourth modificationexample. FIG. 17 is a schematic configuration diagram showing amountingstructure of an optical system included in a laser interferometeraccording to a fifth modification example. FIG. 18 is a schematicconfiguration diagram showing a mounting structure of an optical systemincluded in a laser interferometer according to a sixth modificationexample.

Hereinafter, modification examples will be described, but in thefollowing description, the differences from the above-describedembodiment will be mainly described, and the description of the samematters will be omitted. In FIGS. 13 to 18, the same reference numeralsare given to the same configurations as those of the above-describedembodiment.

An optical system 50A of a laser interferometer 1 shown in FIG. 13 isthe same as the optical system 50 shown in FIG. 2, except that thearrangement of the optical path length variable section 13 is different.

In the above-described optical system 50 shown in FIG. 2, the opticalpath length variable section 13 is arranged on the optical path 22between the polarization beam splitter 4 and the object 14 to bemeasured. In contrast, in the optical system 50A shown in FIG. 13, theoptical path length variable section 13 is arranged on the optical path20 between the polarization beam splitter 4 and the optical modulator12.

According to such a configuration, since the optical path lengthvariable section 13 can be arranged between the polarization beamsplitter 4 and the optical modulator 12, although it depends on theconfiguration of the optical system 50A, a laser interferometer 1 usefulfrom the viewpoint of ease of manufacture and maintainability for theoptical path length variable section 13 can be obtained.

An optical system 50B of a laser interferometer 1 shown in FIG. 14 isthe same as the optical system 50 shown in FIG. 2, except that theconfiguration of an optical path length variable section 13A isdifferent.

The above-described optical path length changing method of the opticalpath length variable section 13 shown in FIG. 2 is a method of changingthe optical path length by moving an optical component. In contrast, theoptical path length changing method of the optical path length variablesection 13A shown in FIG. 14 is a method of changing the refractiveindex of the medium through which the optical path 22 passes.

Specifically, the optical path length variable section 13A shown in FIG.14 includes a variable refractive index medium 132A and a temperaturecontrol portion 138A that changes the temperature of the variablerefractive index medium 132A. The variable refractive index medium 132Ais a light-transmitting medium arranged on the optical path 22, and hasa characteristic that the refractive index changes with a temperaturechange.

According to such a configuration, the optical path length of theoptical path 22 can be changed without changing the physical distance ofeach portion constituting the optical path length variable section 13A.Therefore, in the optical system 50B, it is not necessary to provide aspace for the volume change of the optical path length variable section13A, so that the laser interferometer 1 can be further miniaturized.

Examples of the medium constituting the variable refractive index medium132A include an inorganic material such as quartz glass and an organicmaterial such as acrylic resin. The variable refractive index medium132A may have a heat insulating structure that enhances the heatinsulating property between the medium and the outside air, ifnecessary. Examples of the temperature control portion 138A include aheat exchange element such as a Peltier element.

The optical path length variable section 13A shown in FIG. 14 isarranged between the polarization beam splitter 4 and the object 14 tobe measured. Therefore, although it depends on the configuration of theoptical system 50B, a laser interferometer 1 useful from the viewpointof ease of manufacture and maintainability for the optical path lengthvariable section 13A can be obtained.

An optical system 50C of a laser interferometer 1 shown in FIG. 15 isthe same as the optical system 50B shown in FIG. 14, except that thearrangement of the optical path length variable section 13A isdifferent.

In the above-described optical system 50B shown in FIG. 14, the opticalpath length variable section 13A is arranged on the optical path 22between the polarization beam splitter 4 and the object 14 to bemeasured. In contrast, in the optical system 50C shown in FIG. 15, theoptical path length variable section 13A is arranged on the optical path20 between the polarization beam splitter 4 and the optical modulator12.

As described above, the laser interferometer 1 shown in FIG. 15 includesa light source 2, an optical modulator 12, a photodetector 10, and anoptical path length variable section 13, and the optical path lengthvariable section 13 changes the optical path length of the optical path20 through which the reference light L2 propagates.

According to such a configuration, in addition to the above-mentionedeffect, the effect that the optical path length variable section 13A canbe arranged between the polarization beam splitter 4 and the opticalmodulator 12 can be obtained. Therefore, although it depends on theconfiguration of the optical system 50C, a laser interferometer 1 usefulfrom the viewpoint of ease of manufacture and maintainability for theoptical path length variable section 13A can be obtained.

An optical system 50D of a laser interferometer 1 shown in FIG. 16includes a substrate 39. A light source 2, an optical modulator 12, anda photodetector 10 are each mounted on the substrate 39. Then, on thesubstrate 39 shown in FIG. 16, the photodetector 10, the light source 2,and the optical modulator 12 are arranged in this order along thedirection orthogonal to the optical path 22 shown in FIG. 15.

Further, the optical system 50D shown in FIG. 16 includes prisms 40 and42. The prism 40 is provided on an optical path 24 between thephotodetector 10 and an optical analyzer 9. The prism 42 is provided onan optical path 20 between the optical modulator 12 and a quarter waveplate 8.

Further, the optical system 50D shown in FIG. 16 includes a convex lens44. The convex lens 44 is provided on an optical path 18 between thelight source 2 and a polarization beam splitter 4. By providing theconvex lens 44, emission light L1 emitted from the light source 2 can befocused and effectively used.

The optical path length variable section 13 shown in FIG. 16 is arrangedon the optical path 22 between the polarization beam splitter 4 and theobject 14 to be measured.

An optical system 50E of a laser interferometer 1 shown in FIG. 17 isthe same as the optical system 50D shown in FIG. 16, except that thearrangement of the elements and the like is different.

On a substrate 39 shown in FIG. 17, a light source 2, a photodetector10, and an optical modulator 12 are arranged in this order in thedirection orthogonal to an optical path 22 shown in FIG. 15. A prism 40is provided on an optical path 18, and a prism 42 is provided on anoptical path 20.

The optical path length variable section 13 shown in FIG. 17 is arrangedon the optical path 22 between the polarization beam splitter 4 and theobject 14 to be measured.

An optical system 50F of a laser interferometer 1 shown in FIG. 18 hasan arrangement in which an optical modulator 12 is incorporated in anoptical path connecting an object 14 to be measured and a photodetector10.

On a substrate 39 shown in FIG. 18, a light source 2, an opticalmodulator 12, and a photodetector 10 are arranged in this order in thedirection orthogonal to an optical path 22 shown in FIG. 15. A prism 40is provided on an optical path 18, and a prism 42 is provided on anoptical path 24.

The optical path length variable section 13 shown in FIG. 18 is arrangedon the optical path 22 between the polarization beam splitter 4 and theobject 14 to be measured.

According to the mounting structures shown in FIGS. 16 to 18 asdescribed above, the size of the laser interferometer 1 can be easilyreduced. The arrangement of the elements is not limited to thearrangement shown in the drawings.

In the mounting structures shown in FIGS. 16 to 18, the size of thephotodetector 10 is, for example, 0.1 mm square, the size of the lightsource 2, is, for example, 0.1 mm square, and the size of the opticalmodulator 12 is, for example, 0.5 to 10 mm square. The size of thesubstrate 39 on which these elements are mounted is, for example, 1 to10 mm square. Thereby, the size of the laser interferometer 1 can bereduced to about the size of the substrate 39.

Even in the above-mentioned modification example, the same effect asthat of the above-described embodiment can be obtained.

Although the laser interferometer according to the present disclosurehas been described above based on the illustrated embodiment, the laserinterferometer according to the present disclosure is not limited to theabove-described embodiment, and the configuration of each section can bereplaced with any configuration having the same function. Further, anyother component may be added to the laser interferometer according tothe embodiment. Moreover, the embodiment of the present disclosure mayinclude any two or more of the embodiment and each of the modificationexamples.

What is claimed is:
 1. A laser interferometer comprising: a light sourcethat emits first laser light; an optical modulator that includes avibrator and modulates the first laser light by using the vibrator togenerate second laser light including a modulated signal; aphotodetector that receives interference light between third laser lightincluding a sample signal generated by reflecting the first laser lighton an object and the second laser light to output a light receptionsignal; and an optical path length variable section that changes anoptical path length of an optical path through which the third laserlight propagates.
 2. The laser interferometer according to claim 1,further comprising an optical path branching element that branches anoptical path through which the first laser light propagates, wherein theoptical path length variable section is disposed on an optical pathbetween the optical path branching element and the object.
 3. A laserinterferometer comprising: a light source that emits first laser light;an optical modulator that includes a vibrator and modulates the firstlaser light by using the vibrator to generate second laser lightincluding a modulated signal; a photodetector that receives interferencelight between third laser light including a sample signal generated byreflecting the first laser light on an object and the second laser lightto output a light reception signal; and an optical path length variablesection that changes an optical path length of an optical path throughwhich the second laser light propagates.
 4. The laser interferometeraccording to claim 3, further comprising an optical path branchingelement that branches an optical path through which the first laserlight propagates, wherein the optical path length variable section isdisposed on an optical path between the optical path branching elementand the optical modulator.
 5. The laser interferometer according toclaim 1, wherein the optical path length variable section changes theoptical path length by an optical distance or more, the optical distancecorresponding to a wavelength of the first laser light.
 6. The laserinterferometer according to claim 2, wherein the optical path lengthvariable section changes the optical path length by an optical distanceor more, the optical distance corresponding to a wavelength of the firstlaser light.
 7. The laser interferometer according to claim 3, whereinthe optical path length variable section changes the optical path lengthby an optical distance or more, the optical distance corresponding to awavelength of the first laser light.
 8. The laser interferometeraccording to claim 4, wherein the optical path length variable sectionchanges the optical path length by an optical distance or more, theoptical distance corresponding to a wavelength of the first laser light.9. The laser interferometer according to claim 1, wherein the opticalpath length variable section includes a movable optical element thatchanges the optical path length by moving, and a drive portion thatdrives the movable optical element.
 10. The laser interferometeraccording to claim 3, wherein the optical path length variable sectionincludes a movable optical element that changes the optical path lengthby moving, and a drive portion that drives the movable optical element.11. The laser interferometer according to claim 1, wherein the opticalpath length variable section includes a variable refractive index mediumof which a refractive index changes with a temperature change, and atemperature control portion that changes a temperature of the variablerefractive index medium.
 12. The laser interferometer according to claim3, wherein the optical path length variable section includes a variablerefractive index medium of which a refractive index changes with atemperature change, and a temperature control portion that changes atemperature of the variable refractive index medium.
 13. The laserinterferometer according to claim 1, further comprising a demodulationcircuit that includes a first signal path and a second signal path anddemodulates the sample signal from a first light reception signal thatis the light reception signal passed through the first signal path and asecond light reception signal that is the light reception signal passedthrough the second signal path, wherein the demodulation circuit isconfigured to detect a maximum amplitude of the first light receptionsignal and record the detected maximum amplitude as a first recordingmaximum amplitude when the optical path length is changed by the opticalpath length variable section, detect a maximum amplitude of the secondlight reception signal and record the detected maximum amplitude as asecond recording maximum amplitude when the optical path length ischanged by the optical path length variable section, adjust the maximumamplitude of the first light reception signal based on the firstrecording maximum amplitude, and adjust the maximum amplitude of thesecond light reception signal based on the second recording maximumamplitude.
 14. The laser interferometer according to claim 3, furthercomprising a demodulation circuit that includes a first signal path anda second signal path and demodulates the sample signal from a firstlight reception signal that is the light reception signal passed throughthe first signal path and a second light reception signal that is thelight reception signal passed through the second signal path, whereinthe demodulation circuit is configured to detect a maximum amplitude ofthe first light reception signal and record the detected maximumamplitude as a first recording maximum amplitude when the optical pathlength is changed by the optical path length variable section, detect amaximum amplitude of the second light reception signal and record thedetected maximum amplitude as a second recording maximum amplitude whenthe optical path length is changed by the optical path length variablesection, adjust the maximum amplitude of the first light receptionsignal based on the first recording maximum amplitude, and adjust themaximum amplitude of the second light reception signal based on thesecond recording maximum amplitude.